Index: head/lib/clang/llvm.build.mk
===================================================================
--- head/lib/clang/llvm.build.mk
+++ head/lib/clang/llvm.build.mk
@@ -22,7 +22,9 @@
 CFLAGS+=	-D__STDC_FORMAT_MACROS
 CFLAGS+=	-D__STDC_LIMIT_MACROS
 CFLAGS+=	-DHAVE_VCS_VERSION_INC
-#CFLAGS+=	-DNDEBUG
+.if ${MK_LLVM_ASSERTIONS} == "no"
+CFLAGS+=	-DNDEBUG
+.endif
 
 TARGET_ARCH?=	${MACHINE_ARCH}
 BUILD_ARCH?=	${MACHINE_ARCH}
Index: head/share/mk/src.opts.mk
===================================================================
--- head/share/mk/src.opts.mk
+++ head/share/mk/src.opts.mk
@@ -132,6 +132,7 @@
     LLD \
     LLD_BOOTSTRAP \
     LLD_IS_LD \
+    LLVM_ASSERTIONS \
     LLVM_COV \
     LLVM_TARGET_ALL \
     LOADER_GELI \
Index: head/tools/build/options/WITHOUT_LLVM_ASSERTIONS
===================================================================
--- head/tools/build/options/WITHOUT_LLVM_ASSERTIONS
+++ head/tools/build/options/WITHOUT_LLVM_ASSERTIONS
@@ -0,0 +1,2 @@
+.\" $FreeBSD$
+Set to disable debugging assertions in LLVM.
Index: head/tools/build/options/WITH_LLVM_ASSERTIONS
===================================================================
--- head/tools/build/options/WITH_LLVM_ASSERTIONS
+++ head/tools/build/options/WITH_LLVM_ASSERTIONS
@@ -0,0 +1,2 @@
+.\" $FreeBSD$
+Set to enable debugging assertions in LLVM.